Early Vaccination of Chickens Induces Suboptimal Immunity Against Infectious Bronchitis Virus

Infectious bronchitis virus (IBV) is highly prevalent in broiler chickens despite extensive vaccination commonly conducted early after hatch. The effects of early vaccination on immune responses were further investigated in chickens primed at increasing ages, followed by booster vaccination with an attenuated Arkansas (Ark) Delmarva Poultry Industry-type vaccine. Results show that vaccination on day 1 of age elicits significantly lower systemic and mucosal antibody responses compared with vaccination at later time points in the life of the chicken. The increase of IBV antibodies in serum from secondary responses after booster vaccination was more dramatic and significantly higher when measured by an Ark spike subunit 1 protein ELISA compared with measuring by non-Ark serotype whole-virus ELISA, which underlines the immunogenic importance of the virus spike at inducing antibodies. However, the levels achieved following boosting did not differ significantly between ages of priming. Thus, it seems that the booster vaccination mitigated the differences detected after prime immunization. In contrast to the continued rise of systemic antibodies after booster vaccination, the levels of mucosal IBV-specific immunoglobulin A decreased after booster vaccination. The recruitment or expansion of cluster of differentiation (CD)4⁺, CD8⁺, and CD4⁺/CD8⁺ T-cell populations in different immune effector sites was increased with age, but remained unaltered by IBV vaccination. In contrast, peripheral blood CD4⁺ cells showed a significant increase in IBV-vaccinated chickens compared with nonvaccinated age-matched controls both after primary and booster immunization. The results of the current study confirm that IBV vaccination on the day of hatch induces suboptimal IBV immune responses both in the systemic and mucosal compartments. This routine practice may be contributing to the immunologic escape of the virus and increased persistence of vaccine virus in vaccinated chickens. However, booster vaccination seems to overcome poor initial responses.